The present invention relates to monitoring the condition of a fluid such as Diesel engine lubricant while the engine is running and providing an electrical signal indicative of the condition of the fluid insofar as its ability to continue its function to effectively lubricate the running engine.
A known technique for monitoring engine lubricant during real time operation is that employing impedance spectroscopy wherein a pair of electrodes are immersed in the lubricant sump and one electrode excited with a low level alternating current voltage at a relatively high and then sequentially at a relatively low frequency and from the current measurements at both frequencies the impedance is computed and the differential impedance correlated with known properties of the fluid at various stages of lubricant stressing. The differential impedance may then be compared with the known values in real time to determine from a lookup table of the differential impedance versus known fluid conditions what the instantaneous condition of the fluid is during engine operation.
Such a lubricant monitor employing impedance spectroscopy techniques is shown and described in U.S. Pat. Nos. 6,278,281, 6,377,052, 6,380,746 and 6,433,560 which are assigned to the assignee of the present invention. The aforesaid '281 patent particularly describes a technique for utilizing impedance spectroscopy to determine the presence of engine coolant in automatic transmission fluid and in passenger car motor oil.
U.S. Pat. No. 6,377,052 describes a method of monitoring synthetic motor oil of the type employed in spark ignition gasoline fueled engines in passenger cars utilizing impedance spectroscopy for determining during real time operation the instantaneous remaining useful life (RUL) of the engine lubricant by comparison of impedance calculated from sequential current measurements at different frequencies for engine oil of a known condition.
However, it has been desired to also monitor, during real time engine operation the condition of lubricant in compression ignition or Diesel engines. In Diesel engines, the lubricant is blended by the lubricant manufacturer with unique additive systems or constituents to prevent deterioration of the lubricant due to the effects of the products of combustion of Diesel fuel. Thus Diesel engine lubricant is formulated with additives having a substantially different chemical composition from that of lubricant formulated for spark ignition engines burning gasoline or other fuels. Accordingly, it has been desired to find techniques and ways of employing impedance spectroscopy to accurately provide an electrical indication, during real time engine operation, of the condition of the lubricant in Diesel engines.